From the Super Mario Wiki
The Ceremonial Chamber is a room in the basement of Treacherous Mansion in Luigi's Mansion: Dark Moon. Accessible via the Guard Tunnel and Haunted Catacombs, this room is one of the main destinations in the first mission of the mansion.
The main part of this room is a circular chamber with numerous torches held by gargoyles, and suits of armor inside, one of which is initially sitting on a throne. Some of these suits of armor are inhabited by Strong Greenies and attack Luigi after he lights the torches. Examining the throne brings Luigi to the Haunted Catacombs.
There's also a corridor to the right with a flame, a giant spider web and another suit of armor. The door here leads to the Cliffside.
This room can also be seen into via the hole in the Guard Tunnel's north wall.
During Front-Door Key, Luigi chases the Polterpup through this room. However, the Polterpup dives through the floor to reach the Haunted Catacombs, meaning he has to light up the torches to reveal the throne. After this, Luigi had to defeat the suits of armor before going to the next area.